This invention relates generally to a method of fabricating an insulated conductor, and more particularly to a method of extruding over a metallic conductor a low loss, unsintered polytetraflouroethylene (PTFE) dielectric core surrounded by a sintered PTFE skin.
Polytetraflouroethylene (PTFE) is typically used as an electrical insulator over metallic conductors in the form of coaxial cables or other types of electrical conductors. PTFE has an extremely high melt viscosity. Consequently, PTFE cannot typically be melt processed. Instead, the PTFE to be processed is typically a fine powder. The powder is mixed with a processing or extruding aid of about 15% to 20% by weight relative to the PTFE. The extruding aid is typically an oil such as, for example, Naphtha. For wire applications, the resulting slurry is then forced by paste extrusion onto and to thereupon coat a conductor. This product is then processed through a vaporizing oven at about 450xc2x0 F. to vaporize the extruding aid out of the PTFE. The product is then further processed through a sintering oven wherein the PTFE is heated above its sintering point of about 700xc2x0 F. When the product exits the sintering oven, the fully sintered PTFE congeals into a solid, homogenous mass. The paste extrusion, vaporizing oven and sintering oven are usually implemented in one continuous process to treat the product. The material is pulled through this process by conventional wire and cable equipment. The velocity of propagation (VP) of the resulting product is about 70% of the speed of light.
During the above-described process, there are typically required several pieces of equipment to pull the coated conductor. The equipment pieces exert their pulling force on the paste extruded PTFE. The PTFE is fully sintered, and unfortunately has a larger attenuation at high operating frequencies of, for example 18 GHz or higher, as compared to unsintered PTFE.
It is a general object of the present invention to provide a method of extruding PTFE over a metallic conductor and product derived therefrom that avoids the above-mentioned drawbacks.
In a first aspect of the present invention, a method of forming a dielectric insulation layer over a metallic conductor of a processed product includes the steps of extruding a generally non-melt processable dielectric such as, for example, polytetrafluoroethylene (PTFE) over a metallic conductor, and heating the dielectric to a temperature substantially below that of the sintering temperature of the dielectric so as to form a sintered dielectric skin over an unsintered dielectric core.
In a second aspect of the present invention, a method of forming a polytetrafluoroethylene (PTFE) insulation layer over a metallic conductor of a processed product includes the steps of extruding PTFE with an extruding aid over a metallic conductor. The extruding aid is vaporized out of the PTFE. The PTFE is heated to a temperature substantially below that of the sintering temperature of PTFE so as to form a sintered PTFE skin over an unsintered PTFE core.
In a third aspect of the present invention, a method of forming a PTFE insulation layer over a metallic conductor of a processed product includes the steps of conveying the processed product at a generally constant speed during a fabrication process. The process includes extruding PTFE with an extruding aid through a die and over a metallic conductor. The extruding aid is vaporized out of the PTFE at a temperature of about 450xc2x0 F. The PTFE is heated to a temperature of about 500xc2x0 F. so as to form a sintered PTFE skin over an unsintered PTFE core.
An advantage of the present invention is that the fabrication process is accomplished at generally the same speed throughout the process.
A second advantage is that a low loss, unsintered PTFE skin is formed compared to a fully sintered PTFE layer.
Other advantages will be made apparent with reference to the description and accompanying drawings.